Oilfield risers are conventionally used in offshore oil drilling operations, and comprise pipe sections intended to surround and isolate casing from the sea water. During makeup and breakout of the riser string, the individual riser sections are generally stacked on the rig floor and are repositioned between a riser storage area and a staging area. Risers typically have one or more fluid conduits or cables exterior of the riser, which complicates the riser handling operation.
U.S. Pat. No. 4,202,653 discloses a pipe handling system including a bridge crane. U.S. Pat. No. 4,808,034 discloses a riser with a ring guide for positioning the riser. U.S. Pat. No. 6,877,281 discloses a stackable riser with a riser cover. None of the above prior art provides an effective system for safely and reliably positioning a riser on a rig between a storage area and a staging area.
Individual riser sections are commonly moved from a staging area to a storage area by a crane. More particularly, a scissors-type tool is conventionally suspended from the crane and is used to grab the riser for manipulation by the crane. In many applications, however, the scissors tool cannot reliably grasp a stacked riser section, and accordingly another initial manipulation mechanism is provided for initially positioning the riser from the stack so it may be subsequently grabbed by the scissors tool. In some cases, this initial manipulation is accomplished by a smaller crane and a series of straps which may be passed under the riser and connected to the smaller crane. Considerable time and expense are involved in using two such cranes to manipulate the riser, and the reliability of the system is a function of the number and correct positioning of the straps, and also the reliability of a scissors-type tool which is merely grasping opposing sides of the riser section.
The disadvantages of the prior art are overcome by the present invention, and an improved lifting tool for supporting an offshore riser during manipulation on a rig is hereinafter disclosed.